1. Field of the Invention
The present invention relates to the art of making an optical head thin in an optical disk drive unit which drives an optical disk used as an information recording medium such as an MD (or mini-disc).
2. Description of the Related Art
With respect to an optical head used for an optical disk drive unit which drives a CD (or compact disc) or the like, conventionally, such a devise has been made smaller and its costs lowered. This has been done by uniting light receiving and emitting elements so that optical parts can be integrated. This art is also applied to an optical head for optical magnetic recording, such as an MD, as disclosed, for example, in Japanese Patent Laid-Open No. 10-143934 specification.
FIG. 11A is a side view of an optical configuration of a conventional optical head in which such light receiving and emitting systems are integrated. FIG. 11B is a top view of the configuration shown in FIG. 11A. In FIG. 11A and FIG. 11B, components have the same reference numerals if they are identical. Hereinafter, the configuration shown in these figures will be described with reference to the figures.
In FIG. 11A, reference numeral 31 denotes an integrated unit, and 32 designates a package used as a case body. Inside of them, a silicon substrate 33 is disposed in which a light emitting element and light receiving elements are mounted or formed. The silicon substrate 33 is sealed in by the package 32 and a hologram element 34. A composite prism 35 is held to the hologram element 34. Reference numeral 36 denotes a light-raising mirror; 37, an objective lens; 38, an optical disk in which an optical magnetic signal is recorded; 39, a cartridge which houses the optical disk 38; and 40, an optical path which leads to the optical disk 38 from a light emitting element on the silicon substrate 33.
In FIG. 11B, reference numeral 41 designates a laser diode as a light emitting element disposed on the silicon substrate 33, and 42, 43, 44 and 45 denote photodiodes as light receiving elements.
Reference numeral and character 34a designates a diffraction grating which is formed in a plane of the hologram element 34. They are disposed in the region which the light emitted from the laser diode 41 passes through.
Reference numerals and characters 35a and 35b denote optical surfaces inside of the composite prism 35. The optical surface 35a, for example, transmits approximately 80 percent of a P-polarized light component, and reflects about 20 percent thereof. However, it reflects nearly 100 percent of an S-polarized light component. In short, its transmission and reflection depend upon polarization. On the other hand, the optical surface 35b reflects almost 100 percent of both a P-polarized light component and an S-polarized light component. Reference numeral and character 35c designates a Wollaston prism which is made of, for example, a birefringence material such as lithium niobate.
In the conventional optical head which has the above described configuration, the hologram element 34 and the composite prism 35 transmit the light emitted from the laser diode 41. Then, the light-raising mirror 36 shifts the direction of the optical path at an angle of approximately 90 degrees. Sequentially, the objective lens 37 converges the light onto the optical disk 38. In the optical disk 38, its polarization is slightly rotated by the Kerr effect. Then, the light is reflected, and again, passes through the objective lens 37. Next, it is incident on the composite prism 35 via the light-raising mirror 36. The optical surface 35a transmits approximately 80 percent of its P-polarization component. About 20 percent of the P-polarization component and almost 100 percent of the S-polarization component generated by the Kerr rotation are reflected.
At the diffraction grating 34a, for example, 10 percent of ± primary diffracted light of the light transmitted by the optical surface 35a is diffracted, respectively. Then, it is received by the photodiodes 42, 43. Although description is omitted of the detailed principle of such a detection, these light reception signals are calculated to obtain a focus error signal and a tracking error signal.
On the other hand, the light reflected by the optical surface 35a is reflected by the reflection surface 35b. Thereafter, it is separated into polarized light components by the Wollaston prism 35c. These components cross at a right angle and are received by the photodiodes 44, 45, respectively. Based on the differential between these light reception signals, an optical magnetic signal is detected.
The conventional optical head which has the above described configuration can be made smaller than the one in which the laser diode 41 is configured as a separate body from the photo diodes 42, 43, 44, 45. In addition, each component can be relatively positioned more precisely, thereby making the device more reliable. Besides, the number-of component parts can be reduced, thus lowering its costs.
Demand for thinner optical disk drive units has been rising in the marketplace. However, the above described conventional configuration shown in FIG. 11A has a disadvantage in that the thickness of the optical head in the thickness direction of the optical disk 38 cannot be made less than a width H of the package 32. This makes it difficult to thin an optical disk drive unit further.
Especially, in the case of a lead frame-type package, the number and pitch of lead wires determine the minimum value of the width H. For example, assuming that the number of lead wires is 14 and the pitch of lead wires is 0.5 mm, the limit of the width H is approximately 3.5 mm. If you take into account the clearance between the optical head and the cartridge, then the thickness of the optical head, or the distance from the lower surface of the cartridge to the lower surface of the optical head, about 4 mm at the minimum.
If you consider only the configuration, then the optical head can be made thin by shortening the distance between the integrated unit 31 and the light-raising mirror 36 and disposing the integrated unit 31 inside of the window of the cartridge 39. However, if the distance between the integrated unit 31 and the light-raising mirror 36 is shortened as described above, the optical-path length between them becomes short. This will raise problems, such as increasing an off-axis aberration when the objective lens 37 is shifted in a tracking operation.